Production method of composite particles

ABSTRACT

There is provided a production method of composite particles, by which the fine composite particles which contain fine fibers in the particles, are spherical and have a particle size of 1 μm or less can be stably obtained. It is characterized in that when the composite particles containing fine fibers in the particles are produced, a water-soluble metal salt is dissolved in an aqueous solution in which the fine fibers have been dispersed, and that an alkali which reacts with a metal ion dissolved in the aqueous solution to deposit a metal compound is thereafter added to the aqueous solution while maintaining dispersion of the fine fibers, thereby depositing the composite particles containing the fine fibers and comprising the metal compound.

TECHNICAL FIELD

The present invention relates to a production method of compositeparticles, and more particularly to a production method of compositeparticles containing fine fibers in the particles.

Background Art

Fine fibers such as carbon nanotubes are high in its cohesive force andeasily agglomerated, so that it is extremely difficult to directly addthe fine fibers to a matrix and uniformly disperse them in the matrix.

Accordingly, the fine fibers can be uniformly dispersed in the matrix,for example, by forming composite particles containing the fine fibersin metal particles, adding these composite particles to the matrix, anduniformly dispersing them in the matrix.

Such composite particles can be obtained by a production method ofcomposite particles proposed in the following patent document 1.

In such a production method, an electrolyte in which fine carbon fiberssuch as carbon nanotubes have been dispersed is electrolyzed to depositmetal particles in which the fine carbon fibers have been mixed on acathode electrode, and then, the metal particles deposited are separatedfrom the cathode electrode.

Patent Document 1: PCT International Publication WO2004/094700 Pamphlet

DISCLOSURE OF THE INVENTION

According to the production method proposed in patent document 1,composite particles comprising metal particles in which fine carbonfibers have been uniformly dispersed can be obtained.

Meanwhile, as the composite particles comprising metal particles to beblended with a conductive paste, there have been desired compositeparticles comprising fine metal particles, which are spherical and havea particle size of 1 μm or less. This is because the conductive pastewith which the composite particles comprising such fine metal particlesare blended exhibits good fluidity and can homogenize a coated surfaceto which the conductive paste has been applied.

However, in an electrolytic process employed in the production methodproposed in patent document 1, a metal tends to easily deposit on acathode electrode in dendrite (arborescent) form. Accordingly, althoughit is possible to deposit the composite particles comprising sphericalmetal particles on the cathode electrode by adjustment of electrolysisconditions, the deposited composite particles comprising the sphericalmetal particles are liable to become coarse particles.

The tendency of such particle coarsening is also inhibitable by usingthe cathode electrode of niobium, titanium or the like, or by addingniobium to an electrolyte. However, it is still difficult to obtain thecomposite particles comprising metal particles, which are spherical andhave a particle size of 1 μm or less.

Further, the concentration of an additive in the electrolyte and thelike vary with the electrolytic time, so that it is difficult to controlthe form or particle size of the resulting composite particlescomprising the metal particles.

It is therefore an object of the invention to provide a productionmethod of composite particles, by which the fine composite particleswhich contain fine fibers in the particles, are spherical and have aparticle size of 1 μm or less can be stably obtained.

The present inventors have made a series of studies for achieving theabove-mentioned object, and have added an aqueous sodium hydroxidesolution to an aqueous solution of copper sulfate in which carbonnanotubes are dispersed. As a result, particles comprising copperhydroxide containing the carbon nanotubes have precipitated. Theseprecipitated particles have been reduced with a reducing agent. As aresult, it has become clear that composite particles containing thecarbon nanotubes, having a particle size of 1 μm or less and comprisingspherical copper particles are obtained, thus completing the presentinvention.

That is to say, the present invention is a production method ofcomposite particles, which is characterized in that when the compositeparticles containing fine fibers in the particles are produced, awater-soluble metal salt is dissolved in an aqueous solution in whichthe fine fibers have been dispersed, and that an alkali which reactswith a metal ion dissolved in the above-mentioned aqueous solution todeposit a metal compound is thereafter added to the above-mentionedaqueous solution while maintaining dispersion of the above-mentionedfine fibers, thereby depositing the composite particles containing thefine fibers and comprising the above-mentioned metal compound.

Further, the present invention is also a production method of compositeparticles, which is characterized in that when the composite particlescontaining fine fibers in the particles are produced, a water-solublemetal salt is dissolved in an aqueous solution in which the fine fibershave been dispersed, and that an alkali which reacts with a metal iondissolved in the above-mentioned aqueous solution to deposit a metalcompound is thereafter added to the above-mentioned aqueous solutionwhile maintaining dispersion of the above-mentioned fine fibers, therebydepositing the composite particles containing the fine fibers andcomprising the above-mentioned metal compound, followed by subjectingthe above-mentioned deposited composite particles to reduction treatmentwith a reducing agent for reducing the metal compound, thereby obtainingthe composite particles comprising the metal particles.

In such a present invention, the composite particles comprising themetal particles, which are subjected to the reduction treatment, can bestored without impairing the characteristics of the composite particlescomprising the metal particles by protecting the particles with aprotecting agent so that corrosion acceleration caused by the differencein potential between the metal which forms the above-mentioned metalparticles and the fine fibers is inhibited to be able to keep a reducedstate of the above-mentioned metal.

Further, in order to maintain dispersion of the fine fibers in theaqueous solution, shocks are given to the above-mentioned aqueoussolution, thereby being able to easily disperse the fine fibers in theaqueous solution in the course of forming the composite particles. Asthe shocks given to the aqueous solution, ones due to an ultrasonic waveare preferred.

Furthermore, also when the alkali is added, the fine fibers can beeasily uniformly dispersed in the aqueous solution by giving the shockto the aqueous solution. When the fine fibers are dispersed in theaqueous solution, a dispersing agent may be added to the aqueoussolution.

As the fine fibers used in the present invention, there can be suitablyused fine fibers having a diameter of 1 μm or less and a ratio of lengthto diameter (aspect ratio) of 2 or more, and as the water-soluble metalsalt, there can be suitably used a water-soluble metal salt comprisingcopper, nickel or silver.

As such fine fibers, there can be suitably used carbon nanotubes.

ADVANTAGE OF THE INVENTION

According to the present invention, the composite particles comprisingthe metal particles, which are deposited with the fine fibers contained,can be easily obtained.

Further, in the present invention, the composite particles comprisingthe metal particles can be obtained by subjecting the compositeparticles comprising the metal compound, which are deposited with thefine fibers contained, to the reduction treatment with the reducingagent.

Such composite particles obtained by the present invention can providefine composite particles which are spherical, have a particle size of 1μm or less, and have not been obtained by an electrolytic processemployed in a conventional production method of the composite particles.

Moreover, in the present invention, the fine composite particles whichare spherical and have a particle size of 1 μm or less can be stablyobtained by controlling the amount of the fine fibers, the amount of thewater-soluble metal salt and the amount of the additive for forming aslightly soluble metal salt or a slightly soluble metal oxide, added tothe aqueous solution.

For this reason, the composite particles obtained by the presentinvention can be suitably incorporated, for example, in a conductivepaste. The conductive paste in which these composite particles areincorporated exhibits good fluidity and can homogenize a coated surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing one example of compositeparticles comprising metal particles obtained by a production methodrelating to the present invention.

FIG. 2 is an electron micrograph showing another example of compositeparticles comprising metal particles obtained by a production methodrelating to the present invention.

FIG. 3 is a traced drawing in which an electron micrograph showing stillanother example of composite particles comprising metal particlesobtained by a production method relating to the present invention hasbeen traced.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the water-soluble metal salt is firstdissolved in the aqueous solution in which the fine fibers have beendispersed. As such fine fibers, there can be used fine fibers having adiameter of 1 μm or less and a ratio of length to diameter (aspectratio) of 2 or more. Specifically, they include fine carbon fibers suchas carbon nanotubes and carbon nanofibers, fine silica fibers, finetitanium fibers and fine resin fibers.

Further, dispersion of such fine fibers can be performed by givingshocks due to an ultrasonic wave to the aqueous solution, or adding adispersing agent while stirring the aqueous solution by mechanicalstirring with a stirrer or the like. The dispersing agents includeoctylphenoxypolyethoxyethanol, sodium dodecylsulfate and polyacrylicacid as surfactants.

In order to perform more easily such dispersion of the fine fibers, itis preferred to give shocks due to an ultrasonic wave to the aqueoussolution to which the above-mentioned dispersing agent has been added.

Further, as the water-soluble metal salt, there can be suitably used awater-soluble metal salt comprising copper, nickel or silver, and morepreferably, there can be used a sulfate, a nitrate or an acetatecomprising copper, nickel or silver.

When the water-soluble metal salt comprising copper, nickel or silverwas used as such a water-soluble metal salt, a hydroxide of copper ornickel, or an oxide of silver is deposited by the reaction with analkali.

Then, the alkali which reacts with a metal ion dissolved in the aqueoussolution to deposit the metal compound is added to the aqueous solutionwhile maintaining dispersion of the fine fibers.

Such a metal compound deposited by adding the alkali forms finecomposite particles while incorporating therein the fine fibersdispersed in the aqueous solution. Accordingly, also when the depositedcomposite particles comprising the metal compound are formed, dispersionof the fine fibers in the aqueous solution is maintained, and finecomposite particles which are deposited in the aqueous solution and inthe course of formation are allowed to be dispersed in the aqueoussolution, thereby being able to obtain the composite particles in whichthe fine fibers are uniformly dispersed.

Such dispersion of the fine fibers and the fine composite particles inthe course of formation in the aqueous solution is possible by givingshocks to this aqueous solution. The shocks can also be given bystirring the aqueous solution by mechanical stirring with a stirrer orthe like. In particular, it is preferred that the shocks due to anultrasonic wave are given to the aqueous solution in which thedispersing agent has been added.

The alkalis used herein include sodium hydroxide, potassium hydroxideand calcium hydroxide.

Further, in order to prevent the coagulation of the deposited finecomposite particles comprising the metal compound, a surfactant may beadded to the aqueous solution.

The thus-deposited fine composite particles comprising the metalcompound are composite particles which are substantially spherical, andcontain the fine fibers having a particle size of 1 μm or less.

Further, such composite particles are formed in the aqueous solution inwhich the fine fibers have been dispersed, and the fine fibers dispersedin the aqueous solution can be incorporated in the composite particlesin the course of forming the composite particles. The fine fibers arecontained in the composite particles formed, in a uniformly dispersedstate.

Such composite particles are separated from the aqueous solution, andeasily uniformly blended with a conductive paste or the like. The finefibers contained in the composite particles can also be uniformlydispersed in a matrix.

In addition, the composite particles may be blended with the conductivepaste or the like in a colloidal state without being separated from theaqueous solution.

Meanwhile, composite particles comprising metal particles, which aremore improved in characteristics such as conductive characteristics thanthe composite particles comprising the metal compound, can be obtainedby subjecting the resulting composite particles to reduction treatmentwith a reducing agent for reducing the metal compound.

As such a reducing agent, there can be used one or two or more kinds ofthe group consisting of hydrazine, a hydrazine compound, formalin,acetaldehyde, formic acid, Rochelle salt, hydroxylamine, glucose andhydrogen peroxide. This reducing agent may be added to the aqueoussolution in which the deposited composite particles comprising the metalcompound are precipitated, or may be brought into direct contact withthe composite particles comprising the metal compound, which has beenseparated from the aqueous solution, thereby reducing the metalcompound. Thus-obtained composite particles comprising the metalparticles, which are subjected to the reduction treatment, are compositeparticles comprising the metal and the fine fibers, so that when thepotential of the metal is baser than the potential of the fine fibers,there is the possibility of corrosion such as oxidation or sulfurationof the metal being accelerated by contact with the aqueous solution orthe air, compared with particles formed by the metal element.Accordingly, the composite particles comprising the metal particles canbe protected in a state in which the reduction treatment has beenperformed, by protecting the composite particles with a protecting agentso as to be able to maintain the reduced state of the metal.

Further, when foaming occurs by the reduction treatment with thereducing agent added to the aqueous solution, or by the surfactantadded, a defoaming agent such as an alcohol may be added.

The resulting composite particles comprising the metal particles can beused as materials such as powder metallurgy, batteries, chemicals,electromagnetic shields, conductive materials, metal bonds for thermalconductive material, friction material contacts, resin fillers andsliding materials, as well as conductive pastes.

EXAMPLE 1

Multilayer carbon nanotubes (0.21 g) having a diameter of severalnanometers as fine fibers, 132 g of purified water and 0.5 g ofoctylphenoxypolyethoxyethanol [trade name: TORITON X-100 (manufacturedby INC Biomedical, Inc.)] as a surfactant were subjected to dispersiontreatment by an ultrasonic homogenizer (VC-750 manufactured by UltraSonic, Inc.), and then, 28 g of copper sulfate pentahydrate (CuSO₄.5H₂O)was put therein, followed by stirring with a stirrer to obtain adispersion.

Further, there were prepared an alkali solution in which 9 g of sodiumhydroxide (NaOH) was added to 102 g of purified water, and a reducingagent solution in which 12 g of hydrazine monohydrate (N₂H₄.H₂O) wasadded to 133 g of purified water.

Then, the alkali solution was added to the resulting dispersion whilegiving an ultrasonic wave with an ultrasonic washer (US-1 manufacturedby as One Co., Ltd.) and stirring with a glass rod. The dispersionbecame a deposition solution in which composite particles comprisinghydroxide of copper were deposited.

To this deposition solution, 50 g of ethanol as a defoaming agent wasadded, and 1.8 g of a corrosion inhibitor (Cu—K manufactured by YukaSangyo Co., Ltd.) as a protecting agent for the composite particlescomprising the metal particles was added, followed by heating up to 60°C.

Further, the reducing agent solution was added with stirring thedeposition solution heated to perform a reduction reaction. In thatcase, 50 g of ethanol was further added depending on the situation offoaming to terminate the reduction reaction. After the reductionreaction was terminated, the deposition solution was cooled to ordinarytemperature, and a precipitate was collected, followed by washing anddrying under vacuum.

The resulting composite particles comprising the metal particles showeda copper color, and when observed under an electron microscope (×40000magnification), they were spherical and had a particle size of 1 μm orless, as shown in FIG. 1.

EXAMPLE 2

Multilayer carbon nanotubes (0.18 g) having a diameter of severalnanometers as fine fibers, 100 g of purified water and 0.4 g ofoctylphenoxypolyethoxyethanol [trade name: TORITON X-100 (manufacturedby INC Biomedical, Inc.)] as a surfactant were subjected to dispersiontreatment by an ultrasonic homogenizer (VC-750 manufactured by UltraSonic, Inc.), and then, 28 g of nickel chloride (NiCl₂) was put therein,followed by heating up to 50° C. while stirring with a stirrer to obtaina dispersion.

Further, there was prepared an alkali solution in which 13 g of sodiumhydroxide (NaOH) was added to 50 g of purified water.

Then, the alkali solution was added to the resulting dispersion whilegiving an ultrasonic wave with an ultrasonic washer (US-1 manufacturedby as One Co., Ltd.) and stirring with a glass rod. The dispersionbecame a deposition solution in which composite particles comprisinghydroxide of nickel were deposited.

Hydrazine monohydrate (N₂H₄.H₂O) (64 g) was added as a reducing agentwhile heating this deposition solution up to 60° C. and stirring with astirrer to perform a reduction reaction. In that case, 100 g of ethanolwas added depending on the situation of foaming to terminate thereduction reaction. After the reduction reaction was terminated, thedeposition solution was cooled to ordinary temperature, and aprecipitate was collected, followed by washing and drying under vacuum.

The resulting composite particles comprising the metal particles showeda nickel color, and when observed under an electron microscope (×18000magnification), they were spherical and had a particle size of 1 μm orless, as shown in FIG. 2.

Further, a traced drawing in which an electron micrograph of thesecomposite particles taken at ×45000 magnification has been traced isshown FIG. 3. Respective end portions of multilayer carbon nanotubes 12,12•• are incorporated in metal particles 10.

Furthermore, the resulting composite particles comprising the metalparticles were immersed in diluted nitric acid to dissolve nickelforming the composite particles, and then, this nickel-dissolvedsolution was filtered through a membrane filter. As a result, themultilayer carbon nanotubes remained on the membrane filter. Themultilayer carbon nanotubes were dried, and the weight thereof wasmeasured. As a result, the weight of the multilayer carbon nanotubescontained in the resulting composite particles was 2.7% by weight.

As apparent from this dissolution experiment and FIG. 3, it is provedthat the multilayer carbon nanotubes are contained in the metalparticles.

EXAMPLE 3

Multilayer carbon nanotubes (0.05 g) having a diameter of severalnanometers as fine fibers, 100 g of purified water and polyacrylic acid(molecular weight: 5000) as a surfactant were added and subjected todispersion treatment by an ultrasonic homogenizer (VC-750 manufacturedby Ultra Sonic, Inc.), and then, 10 g of silver nitrate (AgNO₃) was puttherein to obtain a dispersion.

Further, there was prepared an alkali solution in which 3.2 g of sodiumhydroxide (NaOH) was added to 50 g of purified water.

Then, the alkali solution was added to the resulting dispersion whilegiving an ultrasonic wave with an ultrasonic washer (US-1 manufacturedby as One Co., Ltd.) and stirring with a glass rod. The dispersionbecame a deposition solution in which composite particles comprisingdark brown silver oxide particles were deposited.

A precipitate was collected from this deposition solution, followed bywashing and drying under vacuum. The resulting composite particlesshowed a dark brown color, and there were obtained spherical compositeparticles comprising silver oxide, which have a particle size of 1 μm orless, when observed under an electron microscope.

EXAMPLE 4

Multilayer carbon nanotubes (0.05 g) having a diameter of severalnanometers as fine fibers, 100 g of purified water and polyacrylic acid(molecular weight: 5000) as a surfactant were added and subjected todispersion treatment by an ultrasonic homogenizer (VC-750 manufacturedby Ultra Sonic, Inc.), and then, 10 g of silver nitrate (AgNO₃) was puttherein to obtain a dispersion.

Further, there were prepared an alkali solution in which 3.2 g of sodiumhydroxide (NaOH) was added to 50 g of purified water, and a reducingagent solution in which 10 g of hydrazine monohydrate (N₂H₄.H₂O) wasadded to 50 g of purified water.

Then, the alkali solution was added to the resulting dispersion whilegiving an ultrasonic wave with an ultrasonic washer (US-1 manufacturedby as One Co., Ltd.) and stirring with a glass rod. The dispersionbecame a deposition solution in which composite particles comprisingsilver oxide were deposited.

To this deposition solution, a discoloration preventing agent (AG-10manufactured by World Metal Co., Ltd.) as a protecting agent for silverwas added, and then, the reducing agent solution was added with stirringthe deposition solution to perform a reduction reaction. After thereduction reaction was terminated, a precipitate was collected, followedby washing and drying under vacuum.

The resulting composite particles comprising the metal particles showeda silver color, and when observed under an electron microscope, theywere spherical and had a particle size of 1 μm or less.

1. A production method of composite particles, which is characterized inthat when the composite particles containing fine fibers in theparticles are produced, a water-soluble metal salt is dissolved in anaqueous solution in which said fine fibers have been dispersed, and thatan alkali which reacts with a metal ion dissolved in said aqueoussolution to deposit a metal compound is thereafter added to said aqueoussolution while maintaining dispersion of said fine fibers, therebydepositing the composite particles containing the fine fibers andcomprising said metal compound.
 2. The production method of compositeparticles according to claim 1, wherein shocks are given for maintainingthe dispersion of the fine fibers in the aqueous solution.
 3. Theproduction method of composite particles according to claim 2, whereinthe shocks given to the aqueous solution are given by an ultrasonicwave.
 4. The production method of composite particles according to claim1, wherein fine fibers having a diameter of 1 μm or less and a ratio oflength to diameter (aspect ratio) of 2 or more are used as the finefibers.
 5. The production method of composite particles according toclaim 1, wherein a water-soluble metal salt comprising copper, nickel orsilver is used as the water-soluble metal salt.
 6. The production methodof composite particles according to claim 1, wherein carbon nanotubesare used as the fine fibers.
 7. A production method of compositeparticles, which is characterized in that when the composite particlescontaining fine fibers in the particles are produced, a water-solublemetal salt is dissolved in an aqueous solution in which the fine fibershave been dispersed, and that an alkali which reacts with a metal iondissolved in said aqueous solution to deposit a metal compound isthereafter added to said aqueous solution while maintaining dispersionof said fine fibers, thereby depositing the composite particlescontaining the fine fibers and comprising the above-mentioned metalcompound, followed by subjecting said deposited composite particles toreduction treatment with a reducing agent for reducing the metalcompound, thereby obtaining the composite particles comprising the metalparticles.
 8. The production method of composite particles according toclaim 7, wherein the composite particles comprising the metal particlesare protected with a protecting agent so that corrosion accelerationcaused by the difference in potential between the metal which forms theabove-mentioned metal particles and the fine fibers is inhibited to beable to keep a reduced state of the above-mentioned metal.
 9. Theproduction method of composite particles according to claim 7, whereinshocks are given for maintaining the dispersion of the fine fibers inthe aqueous solution.
 10. The production method of composite particlesaccording to claim 9, wherein the shocks given to the aqueous solutionare given by an ultrasonic wave.
 11. The production method of compositeparticles according to claim 7, wherein fine fibers having a diameter of1 μm or less and a ratio of length to diameter (aspect ratio) of 2 ormore are used as the fine fibers.
 12. The production method of compositeparticles according to claim 7, wherein a water-soluble metal saltcomprising copper, nickel or silver is used as the water-soluble metalsalt.
 13. The production method of composite particles according toclaim 7, wherein carbon nanotubes are used as the fine fibers.